Sputtering, alternatively called physical vapor deposition (PVD), is the most prevalent method of depositing layers of metals and related materials in the fabrication of semiconductor integrated circuits. The most commonly practiced form of sputtering is DC magnetron sputtering in which a negative DC voltage is applied to a metallic target in opposition to a grounded chamber shield. A magnetron positioned in back of the target projects a magnetic field adjacent the front face of the biased target to create a high-density plasma of a sputter working gas such as argon. The argon ions of the plasma efficiently sputter metal atoms from the target.
Much of modern sputtering is based on ionizing a large fraction of the sputtered atoms. A high ionization fraction may be accomplished by a small strong magnetron, which intensifies the plasma density and strongly concentrates in area near the magnetron, and by increasing the target power to the 10 kW range and above. DC magnetron sputtering typically generates much heat in the target and high target power further increases the heat. Accordingly, it is standard practice in plasma sputtering to water cool the target. Typically for wafer processing, a cooling bath of chilled water is contained at the back of the target, which requires the magnetron to be placed in the bath. The most commonly used metal targets of aluminum, copper, and even refractory metals such as titanium and tantalum readily conduct the heat generated by the plasma on the front face of the target to the cooling water at the back.
Recently, however, interest has developed in sputtering less conductive metals, metal alloys, metal nitrides, and metal oxides, and semiconductor or related materials including GexSbyTez (germanium antimony telluride or GST). An example of GST is Ge2Sb2Te5 used for non-volatile phase-change memory (PCM). Targets of GST are subject to a strict thermal budget. GST is a metal chalcogenide which is sometimes characterized as a semi-metal rather than a semiconductor. The chalcogen tellurium is in Group VI, as are the chalcogens sulfur and selenium, while germanium and antimony are Group IV and V, the former being an elemental semiconductor and the latter often part of a semiconductor III-V compound. The Groups IV and V components will be considered as metals. A notable property of such metal chalcogenide materials is a possible thermally induced phase change between a resistive amorphous state and a conductive polycrystalline state. The phase change can be induced or written by pulsing current through a GST cell to raise it above a transition temperature. Depending on whether the cell is quickly or slowly quenched to a lower temperature below the transition temperature, the cell upon cooling remains in a resistive amorphous state or changes to a conductive polycrystalline state depending upon the details of the degree of heating and the rapidity of cooling. The memory state can then be read electrically by effectively measuring the resistance of the cell.
Metal nitrides are typically moderately electrically conductive and electrically conductive metal oxides are known which can be used for electrodes.
Although even dielectric materials may be RF sputtered, DC magnetron sputtering is preferred for its speed and low equipment cost and the advanced development of DC magnetron sputtering. Low electrical conductivity is generally though not universally associated with low thermal conductivity according to the Wiedemann-Franz law, which is generally applied to metals but not oxides or nitrides. In general, materials other than highly conductive metals poorly conduct heat and thus are subject to excessive heating in conventional sputtering, which may lead to target degradation including cracking. Non-catastrophic effects include a strong temperature profile across the target, which may degrade sputtering uniformity.